it depends on when the story takes place. my money is that episode vii is gonna take place about 30-ish years after episode vi. that way they can bring back as many of the original actors as they can (assuming they're game to come back - many have already said they would).

fans are way too overprotective of these characters and recasting just sets them up for impossible obstacles. it'd be much easier to get fans to accept brand new characters with no expectation, than accepting new actors replacing what everyone's familiar with.